There are 14 known mammalian isoforms of carbonic anhydrases. Each of these isozymes possesses carbon dioxide-hydrating activity that functions to maintain intracellular pH. Carbonic anhydrase isozyme III (CAIII) is distinguished from the other isoforms by several characteristics--particularly, by a low specific activity that is about 1 per cent that of isozyme II. CAIII is not inhibited by acetozolamide, an excellent inhibitor of CAI and CAII. Also, the amount of CAIII is remarkably high in muscle tissue and adipocytes, constituting about 8 and 25 per cent of the soluble protein content of these tissues. Despite numerous publications on the enzyme, the physiological function of CAIII remains unknown. We created a knockout to facilitate investigation of the function of CAIII. Offspring of heterozygote matings were genotyped. The results were clearly Mendelian. Thus far we have experimentally tested two hypotheses for the physiological function of CAIII. The first was that CAIII conferred resistance to oxidative stress. This was tested by exposing mice to 100 % oxygen, which revealed no difference in resistance among wild type, heterozygote, and homozygote. The second hypothesis was that CAIII is essential for normal fat tissue maintenance, since it constitutes 1/3 of the soluble protein in adipocytes. However, fat distribution and amount were normal in the knockouts. Experiments now in progress are designed to assess other possible roles of CAIII, especially in muscle.